Mixing reactors are generally cylindrical and have standard diameters. Typically they are equipped with baffles attached on the reactor walls, with the purpose of removing a central vortex that sucks gas from the surface, which is considered harmful. Solids-solution processes normally require mixing, which includes both powerful turbulence and adequate circulation.
Feeding into the mixing reactor usually takes place by feeding both the solid and the solution into the reaction space from above. Generally it is desired that in a reactor operating continuously both solids and solution be removed more or less at the slurry density of the reaction space. Thus it is not desirable that even the heaviest or coarsest particles remain in the reactor. Thereby it is natural that the removal of the slurry stream can be installed advantageously in the reactor wall, occurring mainly as the overflow.
However, there are cases in which one wishes to remove the solution that has reacted with the solids from the reaction space in pure form, i.e. without solid particles. One such case is presented in U.S. Pat. No. 3,954,452, in which the cementation reaction of cadmium solution and zinc powder is carried out on the fluidized bed principle. When the solution is fed into the lower section of the reaction zone, efficient mixing is achieved in the fluidized bed. The lower part of the reactor widens conically upwards and is cylindrical in shape from that point upwards. There are baffles in the wall of the lower part of the reactor, which in conjunction with the mixing element crush any agglomerates that are generated. The upper part of the reactor also widens conically upwards. The reactor consists thus of three zones: the reaction, calming and clarification zones, whereby the reaction section is the lower part of the reactor, the centre forms the calming section and the upper section the clarification zone. A mixing element to accomplish the actual mixing is not used here.
In the method according to U.S. Pat. No. 3,954,452 the solution from the fluidization zone rises via the conical widening to the clarification zone, where the solution removal unit is on the wall of the clarification section. The process presented is the cementation of cadmium solution and zinc powder. In this cementation reaction cadmium powder is formed, which is lighter due to its porosity and at the same time also finer. One purpose is to prevent the solid particles formed as reaction product from exiting the reactor with the solution. Another difficulty found in this case has also been the sticking to each other of the barb-like particles i.e. agglomeration. Gradually the agglomerates grow so large that movement in the fluidized bed deteriorates and finally stops completely. For this reason a flocculant solution to prevent agglomeration is fed into the fluidization zone. Since prevention is not completely perfect in practice, a mixing element to crush the agglomerates is located in the lower section and correspondingly smallish baffles are located on the walls to absorb the impact force and prevent vortices.
How strong and how high from the surface of the fluidized bed (Hmax) the upward-directed discharges reach is dependent on the conditions of the fluidization zone. Thus it is important that the stream rises above the aforementioned height as uniformly and at as low a speed as possible.
However, in practice what happens is that the solution flows as directly as possible and by the shortest route towards the removal unit, so that the flow field becomes a tapering curved cone. This in turn means that the speed of the solution stream carrying any possible particles increases and there is no chance for the particles to free themselves from the flow.
The problem with the equipment described above is that the bed material preventing the removal of solids should be fairly coarse. However, as the reactions proceed, the particle size of the solids in the bed decreases, whereupon the amount of solids carried along with the solution increases.